1. Field of the Invention
The present invention relates to a refrigeration cycle apparatus using carbon dioxide as refrigerant and having a compressor, an outdoor heat exchanger, an expander and an indoor heat exchanger.
2. Description of the Related Art
A flow rate of a mass of refrigerant which circulates through a refrigeration cycle apparatus is the same in all points in a refrigeration cycle. If a suction density of refrigerant passing through a compressor is defined as DC and a suction density of refrigerant passing through an expander is defined as DE, the DE/DC (density ratio) is always constant.
In recent years, attention is focused on a refrigeration cycle apparatus using, as a refrigerant, carbon dioxide (CO2, hereinafter) in which the ozone destruction coefficient is zero and global warming coefficient is extremely less than that for Freon. The CO2 refrigerant has a low critical temperature as low as 31.06° C. When a temperature higher than this temperature is utilized, a high pressure side (outlet of the compressor—gas cooler—inlet of pressure reducing device) of the refrigeration cycle apparatus is brought into a supercritical state in which CO2 refrigerant is not condensed, and there is a feature that operation efficiency of the refrigeration cycle apparatus is deteriorated as compared with a conventional refrigerant. Therefore, it is important for the refrigeration cycle apparatus using CO2 refrigerant to maintain optimal COP, and if a temperature of the refrigerant is changed, it is necessary that a pressure is adjusted to a refrigerant pressure which is optimal to the refrigerant temperature.
However, when the refrigeration cycle apparatus is provided with the expander and power recovered by the expander is used as a portion of a driving force of the compressor, the number of rotations of the expander and the number of rotations of the compressor must be the same, and it is difficult to maintain the optimal COP when the operation condition is changed under constraint that the density ratio is constant.
Hence, there is proposed a structure in which a bypass pipe which bypasses the expander is provided, the refrigerant amount flowing into the expander is controlled, and the optimal COP is maintained (see patent documents 1 and 2 for example).
[Patent Document 1]
Japanese Patent Application Laid-open No.2000-234814 (paragraphs (0024) and (0025) and FIG. 1)
[Patent Document 2]
Japanese Patent Application Laid-open No.2001-116371 (paragraph (0023) and FIG. 1)
However, there is a problem that as a difference between a volume flow rate of fluid which flows into the expander and an optimal flow rate in terms of design is increased, an amount of refrigerant flowing through the bypass pipe is increased and as a result, power which could have been recovered cannot be sufficiently recovered.
If the power recovered by the expander is used as a driving force for an auxiliary compressor which is different from the compressor, it is possible to eliminate the constraint that the number of rotations of the expander and the number of rotations of the compressor must be the same. However, even if the auxiliary compressor is driven by the expander, the constraint that the density ratio is constant still remains, and it is still necessary to control the amount of refrigerant which flows into the expander.
Thereupon, it is an object of the present invention to reduce the constraint that the density ratio is constant as small as possible, and to obtain high power recovery effect in a wide operation range.